Ferrite material containing fluorine



June 11, 1963 E. H. FREI ETAL FERRITE MATERIAL CONTAINING FLUORINECERC/VE F0505 Filed April l5, 1959 KEMA/VENCE l United States Pater3,093,453 FERRITE MATERIAL CGNTAINING FLUG Ephraim Heinrich Frei,Michael Schieber, and Shmnel Shtrikman, Rehovot, Israel, assignors toThe Weizmann Institute of Science, Rehovot, Israel Filed Apr. 15, 1959,Ser. No. 806,655 3 Claims. (Cl. 23-50) Our invention relates to novelferrites and more particularly to novel magneto-plumbite ferrites suchas 'barium ferrite in which part of the oxygen of the barium ferrite isreplaced by fluorine, and therefore the trivalent iron is reduced todi-valent iron or can be replaced by another di-valent metal.

The commonly known ferrite used as a magnet is lbariumferrite--B|aO6(Fe2O3). This ferrite has the fadvan-tage of beinginexpensive. It has a high coercive force, yand therefore isincreasingly used in place of steel a Alnico magnets.

However, it has the disadvantage of having low remanence magnetizationdue -to the fact that the molecules of barium ferrite contain only 20non-compensating electron spins which are the basis for the saturationmagnetization of the material.

In laccord-ance with our novel composition of matter, We raise theremanence magnetization of prior ferrites.

Accordingly, an object of our invention is to provide a novelcomposition of matter for increasing remanence magnetization.

A further object of our invention is to provide a novel ferrite forincreasing remanence magnetization.

These and other objects Will 'be clear from the drawing in which theFIGURE is the usual hysteresis loop of magnetizable material.

This ligure shows the hysteresis curves of `a representative Alnico,that means metal magnet, and of barium ferrite. It can be seen that theAlnico magnet has several times higher remanence than the ferrite, buton the other hand the coercive force of the Alnico magnet is much lowerthan the coercive force of the barium ferrite.

As the desirable property of la permanent magnetic material is a highenergy' product i.e. the product of magnetization times eld strengthwhich will result from a high coercive force and remanence respectively,these latter two parameters should be increased as much as possible.

In its turn, remanence depends very much on the saturationmagnetization, it is between the value of saturation magnetization and50% of it. It can 'be seen from the curve that lbarium ferrite vandsimilarly all known ferrite magnets have the drawback of a relativelylo-w saturation magnetization and accordingly therefore of a lowremanence.

We have discovered that by replacing one or a small number of oxygenions of la ferrite by iluorine ions that the saturation magnetizationcould be increased because of the fact that the di-valent oxygen is nowreplaced by only mono-valent fluorine, 'and therefore that tri-valentiron is now either partly being reduced to `di-valent metal or the ironcan be replaced now 'by another di-valent metal of similar ion size. Butif the ferrites already contain divalent metal ions, they can bereplaced by monovalent metal ions when the oxygen ion is replaced byfluorine. For instance, We have made LiFFe2O3 which is derived from theknown FeOFe2O3.

The ferrites are really non-compensated antiferromagnetic orferrima-gnetic materials. This means the magnetization is -only thedifference between la stron-ger magnetization in one direction and aweaker magnetization in the opposite direction.

Clearly, if we can reduce the weaker magnetization in "ice 2 the`opposite direction, then the remaining difference will become largerand therefore the magnetization, both saturation magnetization andremanence magnetization, will now be larger.

This reduction Iof magnetization in the opposite direction will beachieved if we have replaced tri-valent iron by dii-valent iron whichhas less Bohrdmagnetons or with Aanother material having less or noBohr-rnagnetons under the condition that this tri-valent iron wassitting at a site in the crystal Where it contributes to themagnetization into the opposite direction.

As .an example of our invention, the Bohr-magnetons of the previouslymentioned baritun ferrite add up to 20 non-compensated B'ohr-magnetonsper molecule, whereas if two oxygen ions are replaced by uorineaccording to the formula BaFZS (Fe2mO3)2(FeUO), they 4add up to 22Bohrdmagnetons. This material having higher saturation magnetizationdoes have higher remanence magnetization yand therefore can give astronger magnet.

The available Work a magnet can pro-duce depends on the product ofremanence magnetization with coercive force (BH). It can be provedtheoretically that the maximum BH which can be reached in :any materialis (21rIS)2. It is therefore very important to increase the saturationmagnetization (Is) as much as possible. In `our case by increasing thesaturation magnetization from 20 to 22 Bohr-magnetons, a 20% increase of-the energy products might be attainable. Up to now an increase ofremanence magnetization of 8% has been attained.

A class of ferrites of hexagonal crystals structure has been describedby G. H. Jonker in the Proceedings of the XV lth International Congresson Pure and Applied Chemistry, section on mineral chemistry, 1958,117-123. We have replaced in :a number of the compounds described byJonkers, one or a small number of the oxygen ions by the same number offluorine ions, :and at the same time changed tri-valent ions to-di-valent ions .or replaced them by di-valent cobalt ions. In thelatter case we have `obtained compounds which show a preferred directionof magnetization in a plane perpendicular to the hexagonal c axis whichJonkers also describes for some compounds in the above-mentioned paper.

Especially we have replaced two oxygen ions in the barium ferrite, whichis one of the compounds described by Jonkers, but which was well-knownpreviously.

The samples are prepared in the following way. Barium iluoride (Bali-2)100 grams is thoroughly mixed with iron oxide (Fe203) 425 grams in laball mill for 24 hours together with la wetting agent such 'as alcohol.Then the material is -dried Iand pre-tired to a temperature of 1100 C.for 12 hours. Following this .the material is ground again in a ballmill for 24 hours` together with a wetting agent such `as alcohol andthen pressed int-o the desired shape under the inlluence of Ia magneticfield, about 7000 oersteds, under a pressure of about 1/2 ton per quarecentimeter (6000 to 17,000 pounds per .square inch). The material isthen dried carefully in 1a drying oven for two hours at a temperature of120 C. and then sintered in a furnace at a temperature of l280 C. underan atmosphere of thoroughly dry oxygen. The product is then carefullycooled and magnetized. The composition at that stage isBaF25(Fe2O3)2(FeO).

We have also made a similar compound by replacing the di-valent iron inthis compound by di-valent cobalt and have yobtained a composition whichis close to B'aFgS (F6203) The 'barium can be replaced totally orpartially by other ions, especially by strontium, lead, calcium or by acombination of these.

An additional advantage of the fluoride besides the BaF2.5(Fe2O3).2(FeO)3. A barium ferrite having a hexagonal magneto plumbite crystal`structure having the formula References Cited in the file of thispatent UNITED STATES PATENTS 2,736,708 Crowley Feb. 28, 1956 2,762,778Gorter et al Sept. 11, 1956 2,893,830 Brixner July 7, 1959 2,946,753Jonker et al. July 26, 1960 2,962,345 Brixner Nov. 29, 1960 FOREIGNPATENTS 697,219 Great Britain Sept. 16, 1953`

1. A BARIUM FERRITE, HAVING THE FORMULA BAO.6FE2O3, AND HAVING AHEXAGONAL MAGNETO PLUMBITE CRYSTAL STRUCTURE IN WHICH TWO OUT OFNINETEEN OXYGEN ATOMS ARE REPLACED BY FLUORINE.